This invention relates to an optical filter used in a field of optical communication to select a light beam of a specific wavelength from multiple wavelength light components. This invention also relates to a glass substrate for the above-mentioned optical filter. In particular, this invention relates to a WDM (wavelength division multiplexing) optical filter. This invention also relates to a glass substrate for use in such optical filter.
In such WDM (wavelength division multiplexing) communication, communication is carried out by combining light beams having wavelengths slightly different from one another into a combined light beam and, inversely, by splitting or demultiplexing the combined light beam to selectively derive a specific wavelength light beam from the combined light beam. Herein, it is to be noted that the optical filter used for light combination and separation has been called a WDM (Wavelength Division Multiplexing) optical filter. As such a WDM optical filter, there are known WDM optical filters described in JP-A H10-339825 and JP-A H10-512975.
Each of the optical filters described in these publications comprises a glass substrate with a dielectric multilayer film of SiO2, TiO2, Ta2O5, or the like formed thereon. Such a dielectric multilayer film is operable as a band-pass filter (BPF) by giving a function which transmits a particular wavelength light beam or which reflects the particular wavelength light beam. As a rule, the substrate on which the dielectric multilayer film is deposited is formed by a glass material, such as silica.
In the meanwhile, it is recently reported that, in the optical filter of the type, a center wavelength in a pass band is drifted due to variation in temperature. It is also reported that such temperature drift depends upon a thermal expansion coefficient of each of the glass substrate and the dielectric multilayer film (Haruo Takahashi, Applied Optics, Vol. 34[4], pp. 667-675, 1995).
In the above-referenced article, description is made about the fact that a center wavelength within the pass band is drifted or shifted towards a positive direction (namely, a longer wavelength direction) when the thermal expansion coefficient of the glass substrate is smaller than a range determined for thermal properties of the dielectric multilayer such as an expansion coefficient. On the other hand, in case where the thermal expansion coefficient of the glass substrate is excessively large, the drift of the filter center wavelength occurs in a negative direction (shorter-wavelength direction).
If the drift is undesirably large, a filter characteristic, i.e., a transmission wavelength unfavorably varies following the change in operation temperature. In particular, if the bandpass filter is used as a narrow band filter, for example, in an optical multiplexer/demultiplexer used in a wavelength multiplexing transmission technique of optical communication, the influence becomes serious because such a narrow band constraint inevitably restricts a transmission density. Following an increase in a degree of wavelength multiplexing, there arises an increasing demand for an optical filter having a more stable characteristic over the variation in temperature as well as an optical multiplexer/demultiplexer using the same. In order to increase a thermal stability, proposal is made of a technique of controlling the temperature of the optical filter. However, this technique requires a complicated structure. Therefore, the difficulty in assuring a long-term reliability is increased and devices and apparatuses become more expensive.
As described above, the temperature drift of the bandpass peak wavelength constitutes one of factors that obstructs a high density optical communication.
In addition, conventional optical filters are disadvantageous in that peeling off of the multilayer from the glass substrate is liable to occur due to a temperature variation.
Taking the above-mentioned background into consideration, this invention has been created so as to reduce a temperature drift at a center wavelength of a pass band and to thereby avoid peeling off of the dielectric multilayer. More specifically, it is an object of this invention to provide a novel glass substrate which has a desired thermal expansion coefficient and a desired composition. It is another object of this invention to provide an optical filter and an optical multiplexer/demultiplexer both of which has a high reliability and which can reduce a temperature drift at a center wavelength within a pass band.
It is another object of this invention to provide a method of manufacturing glass which has a thermal expansion coefficient pertinent to a substrate material for a wavelength multiplexing/demultiplexing optical filter. Such glass can be obtained by controlling an amount of specific glass components.
According to a first aspect of this invention, a glass substrate is for use in a wavelength multiplexing/demultiplexing optical filter and is formed by glass which includes SiO2 and which has a thermal expansion coefficient between 100xc3x9710xe2x88x927 and 130xc3x9710xe2x88x927/K within a temperature range between xe2x88x9230 and +70xc2x0 C.
According to a second aspect of this invention, a glass substrate is used for an wavelength multiplexing/demultiplexing optical filter and is formed by glass which includes SiO2, R2O (wherein R is representative of an alkali metal element), and TiO2 as essential components and which comprises a total of the essential components not smaller than 60 mol %.
According to a third aspect of this invention, a glass substrate for the wavelength multiplexing/demultiplexing optical filter and is formed by glass which includes SiO2, R2O (wherein R is representative of an alkali metal element), and TiO2 as essential components a total amount of which is greater than an amount of each of the remaining components.
According to a fourth aspect of this invention, the glass mentioned in connection with each of the first through the third aspects comprises, by mol %,
According to a fifth aspect of this invention, the glass mentioned in connection with the fourth aspect comprises, by mol %, as R2O,
According to a sixth aspect of this invention, the glass mentioned in connection with each of the second and the third aspects of this invention comprises, by mol %,
According to a seventh aspect of this invention, the glass mentioned in conjunction with each of the second through the sixth aspects comprises at least one species of oxides RO selected from a group consisting of alkaline earth metal oxides and zinc oxide.
According to an eighth aspect of this invention, the glass mentioned in the seventh aspect comprises, by mol %, a total of RO between 2 and 15%.
According to a ninth aspect of this invention, the glass mentioned in each of the seventh and the eighth aspects comprises, as RO, by mol %
According to a tenth aspect of this invention, the glass mentioned in each of the seventh through the ninth aspects comprises, by mol %,
According to an eleventh aspect of this invention, the glass mentioned in each of the second through the tenth aspects comprises, by mol %,
According to a twelfth aspect of this invention, the glass mentioned in each of the second through the eleventh aspects has an average thermal expansion coefficient between 100xc3x9710xe2x88x927 and 130xc3x9710xe2x88x927 at a temperature range between xe2x88x9230 and +70xc2x0 C.
According to a thirteenth aspect of this invention, the glass mentioned in the twelfth aspect has an average thermal expansion coefficient between 105xc3x9710xe2x88x927 and 120xc3x9710xe2x88x927 at a temperature range between xe2x88x9230 and +70xc2x0 C.
According to a fourteenth aspect of this invention, the glass mentioned in each of the first through the thirteenth aspects has a Knoop hardness not smaller than 455 MPa.
According to a fifteenth aspect of this invention, a wavelength multiplexing/demultiplexing optical filter has a glass substrate mentioned in conjunction with each of the first through the fourteenth aspects and an optical multilayer deposited on the substrate.
According to a sixteenth aspect of this invention, the optical filter has a temperature drift between xe2x88x920.0025 nm/K and +0.0025 nm/K at a center wavelength of a pass band.
According to a seventeenth aspect of this invention, a wavelength multiplexing/demultiplexing optical unit has the optical filter mentioned in each of the fifteenth and the sixteenth aspects.
According to an eighteenth aspect of this invention, a method is for manufacturing glass used in a glass substrate of an optical filter. The optical filter has an optical multilayer which is fixedly deposited on the glass substrate and which has a band pass filter function. The method comprises the step of obtaining the glass which has an average thermal expansion coefficient between 100xc3x9710xe2x88x927 and 130xc3x9710xe2x88x927 at a temperature range between xe2x88x9230 and +70xc2x0 C., by controlling an amount of TiO2 and alkali metal oxide RO as a glass component.
According to a nineteenth aspect of this invention, the method mentioned in the eighteenth aspect adjusts the amount of TiO2 and RO in consideration of a usable temperature range of the optical filter, so that a temperature drift at the center wavelength within the pass band of the optical multiplayer becomes minimum in the usable temperature range.
Description will be made about this invention more in detail.
A glass substrate is used to manufacture an optical filter by successively stacking, on a glass substrate surface, a high refractive index dielectric film and a low refractive index dielectric film and by forming an optical multilayer which has a band pass function passing through a specific wavelength of light within incident light beam, by using optical interference.
As mentioned before, it is necessary to reduce a temperature drift at a center wavelength in a pass band on the wavelength multiplexing/demultiplexing. Herein, it is to be noted that the above-mentioned band pass function can be accomplished by using the optical interference in the multilayer. This means that a reduction of a temperature drift needs to decrease fluctuation among optical lengths within the multilayer due to a temperature variation. It is to be considered that the fluctuation among the optical lengths results from a variation of refractive indexes of each film included in the multilayer and a variation of thicknesses of the films.
In addition, consideration must be also made about the fact that the glass substrate is also thermally expanded or shrunk together with the multilayer, namely, an optical multilayer when the optical filter is exposed to a variation of a temperature. As readily understood from the above, since the optical multilayer is fixedly deposited on the glass substrate surface, thermal stress is imposed onto the optical multilayer due to the thermal expansion or shrinkage of the glass substrate when the glass substrate and the multilayer are different from each other in thermal expansion coefficients. This thermal stress brings about a slight variation of the thickness and the refractive index in the optical multilayer.
Herein, it is assumed that the variation of the thickness and the refractive index of the optical multilayer, that might take place due to the thermal stress can be cancelled by the variation of the thickness that might occur due to the thermal expansion and shrinkage of the optical multilayer. In this event, it is possible to reduce the variation of the optical lengths within the optical multilayer.
As will be mentioned hereinafter in detail, a practical optical multilayer according to this invention makes it possible to cancel both the above-mentioned variations and, as a result, to reduce the variation of the optical lengths within the optical multilayer. Specifically, it has been found out according to the inventors"" experimental studies that the temperature drift can be reduced by rendering an average linear expansion coefficient of the glass substrate into a range between 100xc3x9710xe2x88x927 and 130xc3x9710xe2x88x927/K (preferably, 105xc3x9710xe2x88x927 and 120xc3x9710xe2x88x927/K) within a temperature range between xe2x88x9230 and +70xc2x0 C.
As mentioned before, it may be considered that the stress has to be caused to occur between the glass substrate and a contact surface of the optical multilayer so as to reduce the variation of the optical lengths due to the temperature variation. However, the substrate of glass is softer than the optical multilayer which is operable as the dielectric films. Therefore, the optical multilayer is peeled off from the glass substrate. As a result, it is difficult to obtain a high reliability when the glass substrate is used.
Under the circumstances, the first viewpoint of this invention resides in a glass substrate which comprises SiO2 and which has an average linear thermal expansion coefficient between 100xc3x9710xe2x88x927 and 130xc3x9710xe2x88x927/K (preferably, between 105xc3x9710xe2x88x927 and 120xc3x9710xe2x88x927/K) within a temperature range between xe2x88x9230xc2x0 C. and +70xc2x0 C. The average linear thermal expansion coefficient set into the above-mentioned range makes it possible to reduce the thermal drift at a center wavelength of a pass band. In addition, inclusion of SiO2 serves to enhance a hardness of the glass and provides a glass substrate which prevents an optical multilayer from being peeled off from the glass substrate.
According to the first viewpoint, SiO2 preferably becomes a glass network-former. Judgement can be made about whether or not SiO2 forms the glass network-former in the following manner. At first, when any other components, such as B2O3, P2O5, which are operable as the glass network-formers, are not included in glass, SiO2 may be judged as the glass network-former. On the other hand, when the glass includes any other glass network-formers, such as B2O3, P2O5, or the like, SiO2 may be judged as the glass network-former when an amount of SiO2 is sufficiently greater than that of the other glass network-formers and may be, for example, twice the amount of the latter.
Thus, the inclusion of SiO2 as the glass network-former is helpful to further increase the hardness of the glass substrate and to prevent the optical multilayer from being peeled off from the glass substrate due to the temperature variation. Consequently, the problem of the peeling off can be solved. Preferably, the glass substrate has, in terms of the Knoop hardness, a hardness which is not smaller than 455 MPa, preferably greater than 460 MPa, and more preferably greater than 500 MPa.
A second viewpoint of this invention resides in a glass substrate which has an average linear thermal expansion coefficient between 100xc3x9710xe2x88x927 and 130xc3x9710xe2x88x927/K, preferably, between 105xc3x9710xe2x88x927 and 120xc3x9710xe2x88x927/K and a composition suitable for obtaining a desirable hardness.
Such a glass substrate can be realized by glass which comprises, as essential components, SiO2, R2O (R: alkali metal elements), and TiO2, a total amount of the essential components exceeding 60 mol %. Alternatively, the glass may comprise, as the essential components, SiO2, R2O (R: alkali metal elements), and TiO2 and a total amount of the essential components may exceed each amount of the remaining components other than the essential components. The amount of R2O is indicative of a total amount of the alkali metal oxides.
Hereinafter, the above-mentioned glass will be referred to as SiO2xe2x80x94R2Oxe2x80x94TiO2 system glass. The glass substrate may have a Knoop hardness which is not smaller than 455 MPa, preferably, 460 MPa, and more preferably, 500 MPa, as mentioned in conjunction with the first viewpoint.
In the SiO2xe2x80x94R2Oxe2x80x94TiO2 system glass, SiO2 serves to harden the glass itself and to improve a weather resistance characteristic of the glass. R2O is operable to control the average linear thermal expansion coefficient. Specifically, R2O serves to adjust an average linear thermal expansion coefficient of the SiO2 inclusion glass to a desired average linear thermal expansion coefficient which falls within a predetermined range mentioned above. TiO2 serves to obtain the desired average linear thermal expansion coefficient within the predetermined range and to further excel the weather resistance characteristic.
With the above-mentioned SiO2xe2x80x94R2Oxe2x80x94TiO2 system glass, it is possible to accurately match the average linear thermal expansion coefficient within the predetermined range at a temperature between xe2x88x9230 and +70xc2x0 C., in consideration of an optical mulitlayer deposited on the glass substrate. Such matching can be carried out by controlling a degree of substitution between R2O and TiO2. For example, the degree of substitution between R2O and TiO2 is adjusted so that the temperature drift becomes minimum (namely, closest to zero) within an avaialbe temperature range (for example, a room temperature). As a result, the average linear thermal expansion coefficient of the glass can be adjusted to a desired value. Herein, the degree of substitution between R2O and TiO2 can be controlled by measuring each amount of raw materials and by melting the glass.
It is noted here that the SiO2xe2x80x94R2Oxe2x80x94TiO2 system glass is transparent for a light beam which has a wavelength band between 1.3 and 1.6 m used for optical communication. This means that the above-mentioned system glass has a high quality as the optical glass.
Next, description will be made about amounts of glass components included in the SiO2xe2x80x94R2Oxe2x80x94TiO2 system glass.
As regards SiO2, less than 38 mol % of SiO2 deteriorates the weather resistance characteristic and decrease the hardness of the glass. Consequently, the optical multilayer is liable to be peeled off from the glass substrate. On the other hand, more than 58 mol % of SiO2 makes the average linear thermal expansion coefficient lower than the predetermined range mentioned above and, as a result, undesirably enhances the temperature drift. Taking the above into consideration, the amount of SiO2 falls within a range between 38 and 58 mol %, preferably, between 38 and 50 mol %, and more preferably, between 38 and 48 mol %. The most preferable range of SiO2 is between 42 and 48 mol %.
As regards the alkali metal oxide R2O, when a total amount is less than 15 mol %, it is difficult to obtain the above-mentioned results. On the other hand, more than 40 mol % of R2O is prone to degrade the weather resistance characteristic of the glass. Thus, the total amount of R2O may be restricted to a range between 15 and 40 mol %, preferably between 22 and 32 mol %. It is preferable that the alkali metal oxide R2O may be Na2O and/or K2O. More preferably, both Na2O and K2O may be included in the glass and the alkali metal oxides may desirably consists of only Na2O and K2O. The amount of Na2O preferably falls within a range between 10 and 25 mol %, more preferably within a range between 13 and 25 mol %, most preferably, within a range between 15 and 22 mol %. On the other hand, the amount of K2O preferably falls within a range between 2 and 15 mol %, more preferably between 4 and 15 mol %, and further preferably between 6 and 15 mol %, and most preferably between 6 and 10 mol %.
As regards TiO2, less than 7 mol % of TiO2 degrades the weather resistance characteristic and make it difficult to render the average linear thermal expansion coefficient within the predetermined range. More than 30 mol % of TiO2 makes it difficult to obtain the average linear thermal expansion coefficient falling within the predetermined range. Therefore, the amount of TiO2 falls within a range between 7 and 30 mol %, preferably, between 10 and 25 mol %, further preferably, between 10 and 22 mol %, or between 12 and 22 mol %, and most preferably, between 12 and 20 mol %.
Although Al2O3 is added as an optional component to the SiO2xe2x80x94R2Oxe2x80x94TiO2 system glass, it serves to improve the weather resistance characteristic and to harden the glass. However, more than 12 mol % of Al2O3 make it difficult to obtain the average linear thermal expansion coefficient within the predetermined range. Under the circumstances, the amount of Al2O3 is restricted to a range between 0 and 12 mol %, preferably between 0.5 and 12 mol %, ore preferably between 0.5 and 8 mol %, and further preferably between 1 and 8 mol % or between 2 and 8 mol %, and most preferably between 2 and 6 mol %.
As mentioned before, Al2O3 is added as the optional component to the SiO2xe2x80x94R2Oxe2x80x94TiO2 system glass but it serves to improve the weather resistance characteristic like SiO2 and to harden the glass. Accordingly, Al2O3 may be added as an essential component to the glass. In this event, the glass preferably comprises, by mol %,
In the above-mention glass composition, it is unnecessary to define the total amount of R2O. However, the total amount of R2O may be restricted to a range between 15 and 40 mol %, preferably between 22 and 32 mol %, like in the above-mentioned composition.
According to the second viewpoint, the glass desirably includes, in addition to the above-enumerated components, at least one oxide selected from a group consisting of alkaline earth metal oxides and zinc oxide. Among others, it is preferable that the glass includes MgO and ZnO. Such alkaline earth metal oxides and zinc oxide improve a devitrification resistance property and a fusion property of the glass. Thus, inclusion of divalent components reduces liquidus temperature and facilitates to manufacture and form the glass. In addition, ZnO is effective to harden the glass and to prevent the optical multilayer from being peeled off. When either the alkaline earth metal oxides or the zinc oxide is included, it is preferable that the total amount of the divalent compoents is not less than 2 mol % so as to realize the above-mentioned effects and is not greater than 15 mol % so as not to degrade the weather resistance characteristic.
In order to realize the excellent devitrification resistance property and to avoid the reduction of the weather resistance characteristic, the glass may comprise, by mol %:
More preferably, the glass may comprise, by mol %, 1-13% of MgO and 0.5-10% of ZnO.
The above-mentioned SiO2xe2x80x94R2Oxe2x80x94TiO2 system glass may include a refining agent, such as Sb2O3, an amount of which is desirably restricted to a range between 0 and 0.1 mol %.
In order to improve weather resistance characteristic, the SiO2xe2x80x94R2Oxe2x80x94TiO2 may include at least one of oxides selected from a group consisting of ZrO2, HfO2, La2O3, and Y2O3. An amount of each component, such as ZrO2, HfO2, La2O3, and Y2O3, may be desirably limited to a range between 0 and 1.2 mol % so that the average linear thermal expansion coefficient is not smaller than the predetermined range. The acid resistivity is excellently improved when each of the above-mentioned components is more than 0.2 mol %.
As long as this invention does not depart from its purpose, oxides of, for example, Li, lanthanide, Nb, Ta, W, B, Ga, In, Ge, Sn, Pb, P, Sb, Bi, Te, may be added to an amount of several mol %. Such addition of the oxides is helpful to adjust a refractive index of glass, a glass transition point, and workability. As far as the object of this invention is accomplished, several percents of the oxide components can be replaced by fluoride instead of the oxide components included in the glass.
However, it has been found out according to the inventors experimental studies that most preferable glass composition is specified by a composition of SiO2, Na2O, K2O, TiO2, Al2O3, MgO, ZnO or by a combination of the above-mentioned composition and Sb2O3 added to the composition as the refining agent.
The glass substrate according to the first and the second viewpoints has a greater thermal expansion coefficient than that of a typical glass generally used. Therefore, even if a metal material, such as carbon steel (thermal expansion coefficient being about 120xc3x9710xe2x88x927/K) and stainless steel (the thermal expansion coefficient being about 110xc3x9710xe2x88x927/K), widely used in industry is used as a fixing member, a difference in thermal expansion between the glass substrate and the fixing member is small. Accordingly, optical distortion resulting from stress caused to occur between the glass substrate and the fixing member due to variation in temperature is small. As regards a plastic material, a similar advantage is obtained because, by selecting the degree of polymerization and a bridging agent, use can be made of a material, such as polyethylene, polystyrene, and polymethyl methacrylate, widely used in industry and having a thermal expansion coefficient between 90 and 150xc3x9710xe2x88x927/K.
Let an optical apparatus be structured by using an optical unit which has an optical element formed by the SiO2xe2x80x94R2Oxe2x80x94TiO2 system glass and which is fixed to a fixing member formed by carbon steel, stainless steel (type 410), polyethylene, polystyrene, and polymethyl methacrylate, as mentioned above. The optical apparatus is small in optical distortion against the thermal variation and is therefore excellent in stability. In addition, the weather resistance is excellent so that use is possible in a wide variety of working environment.
Next, description will be made about the optical filter of this invention.
The optical filter of this invention is used for an optical multiplexer/demultiplexer apparatus for wavelength multiplexing/demultiplexing. The optical filter comprises the above-mentioned glass substrate and an optical multilayer formed on the glass substrate by successively depositing and stacking high refractive index dielectric film or films and low refractive index dielectric film or films. The optical mulitlayer has a band pass function resulting from optical interference and can change a center wavelength within a pass band by varying its structure and a refractive index.
As a material for the high refractive index dielectric film, use is made of TiO2, Ta2O5, HfO2, ZrO2, CeO2, Al2O3, Y2O3, ZnS, MgO, La2O3, Cds, Si, or the like. As a material for the low refractive index dielectric film, SiO2, MgF2, ThF4 may be used. A preferable material for the high refractive index dielectric film may be Ta2O5, TiO2 while a preferable material for the low refractive index dielectric film may be SiO2.
The temperature drift of the center wavelength in the optical filter can be reduced by appropriately adjusting the average linear thermal expansion coefficient of a substrate material used. The temperature drift of the center wavelength within the pass band can be suppressed within a range between xe2x88x920.0025 nm/K and +0.0025 nm/K by using the substrate which has an appropriate average linear thermal expansion coefficient within a range between 100xc3x9710xe2x88x927 and 130xc3x9710xe2x88x927/K, although it depends upon the characteristics of the film to be deposited and the depositing conditions. Therefore, the optical filter can be used over a wide temperature range. In addition, the glass substrate material is excellent in weather resistance characteristic and is therefore advantageous in that no problem takes place in connection with a surface degradation which might occur during a polishing process and the like. As a result, the optical filter is available under various working environments.
Herein, it is assumed that wavelength multiplexing/demultiplexing is carried out within a wavelength band of 1.5 xcexcm and that an interval between wavelength components demultiplexed becomes equal to 100 GHz (corresponding to the wavelength interval of 0.8 nm). In addition, it is also assumed that the optical filter has a high transmittance band width of 0.2 nm. Under the circumstances, when the temperature drift exceeds 2.5 pm (0.0025 nm)/K and a temperature is changed over a temperature range of 100 degrees (for example, xe2x88x9230 to +70xc2x0 C.), a signal optical wavelength is shifted from the high transmittance band width to an untransparent region and can not be demultiplexed. Thus, inconvenience takes place in the optical filter.
In addition, let the interval of the wavelength components multiplexed and demultiplexed be equal to 50 GHz (corresponding to the wavelength interval of 0.4 nm). Inasmuch as the optical filter has a high transmittance band width of about 0.1 nm, the signal optical wavelength probably falls within an untransparent region when the temperature drift exceeds 0.5 pm and a temperature variation of about 100xc2x0 C. (for example, xe2x88x9230 to +70xc2x0 C.) is caused to occur in the optical filter. As a result, inconvenience takes place in this case also.
As mentioned before, the optical filter according to this invention has a temperature drift between xe2x88x920.0025 nm/K and +0.0025 nm/K (preferably, between xe2x88x920.0005 nm/K and +0.0005 nm/K). Therefore, it is possible to prevent the signal optical wavelength from being located outside of the high transmittance band width within a usual temperature variation range and to accomplish a high reliability.
Since the glass substrate used in the optical filter has a Knoop hardness not smaller than 455 GPa, it is possible to prevent the optical multilayer from being peeled off from the glass substrate due to a difference of linear thermal expansion coefficients between the optical multilayer and the glass substrate, even when the temperature variation exceeds 100xc2x0 C.
According to this invention, an optical demultiplexer apparatus is obtained which comprises an optical filter, an optical fiber guiding a wavelength multiplexed light beam onto the optical filter through a light exit end of the optical fiber and optical fibers having incident ends to which light wavelength components are given through an optical multilayer of the optical filter.
According to this invention, an optical multiplexer apparatus is obtained which comprises an optical filter having an optical multilayer, a plurality of optical fibers located to the optical filter to conduct wavelength components transmitted or reflected by the optical multilayer, and an optical fiber positioned to conduct the transmitted and the reflected wavelength components and to guide a multiplexed light beam through an incident end of the optical fiber. In consideration of the number of multiplexed wavelengths, a plurality of the optical filters may be used which are different from one anther in center wavelengths of the pass bands and each of which individually carries out multiplexing/demultiplexing operation.
The optical multiplexer apparatus and the optical demultiplxer apparatus (both of which will be often collectively called optical multiplexer/demultiplexer apparatus) have a high reliability even in an environment intensely varied in temperature, because use is made of the optical filter having the high reliability.